All-axis control of aircraft in deep stall

ABSTRACT

Tilting the stabilizer at an extreme angle to the fuselage, with leading edge down, and varying engine thrust comprise a method for all-axis control of a generally conventional aircraft in deep stall. In an alternative embodiment, tilting the engines upward at an extreme angle to the fuselage and varying engine thrust comprise a method for all-axis control in deep stall.

BACKGROUND OF THE INVENTION

This invention relates to fixed wing aircraft and, more particularly, tothe configuration and method for controlling the flight of aircraft whenthe primary lifting surface is in deep stall.

It has generally been considered undesirable in normal operation offixed wing aircraft to operate in deep stall. This condition occurs whenthe primary lifting surface, the wing, is at so positive an anglerelative to the oncoming air flow, that flow line separation occursalong the entire upper wing surface and lifting force is lost. Theresult of such deep stall is generally an uncontrollable drop inattitude of the aircraft's nose section. Control is not regained exceptas air speed is increased by advancing engine thrust and by anaccelerating drop in altitude.

Under the low speed and low altitude conditions of landing, where highwing lift is necessarily produced by flying close to wing stallconditions, the unintended incidence or inducement of deep stall mightleave little time or room for recovery of control with damagingconsequences. The seriousness of the problem is intensified whenlandings are required on short runways or over undeveloped landingareas.

Prior art has concerned itself with the role played by the tailsurfaces, notably the stabilizer, in efficiently developing the highlypositive angle of incidence required of the wing in low-speed flightconditions. The downward force generated at the rear of the fuselage bythe stabilizer is used to rotate the aircraft to the wing's maximumunstalled angle of attack. On conventional airplanes havingsubstantially horizontal stablizers, the downward thrust of the tailsection is developed by upwardly tilting the elevator flaps hinged tothe rear of the fixed stabilizer surface. However, in this slightlynose-up position of the airplane, the raised elevator flaps cause adownward thrust while, concurrently, the horizontal stabilizer sectionfixed in relation to the fuselage, generates increased lift (as comparedto level flight) in opposition to the raised flaps. To overcome thisinefficiency of opposed forces and to increase the down thrust whileusing smaller tail surfaces, fully tiltable stabilizers have beendeveloped where the entire stabilizer surface rotates leading edge down,relative to the level flight axis of the fuselage. U.S. Pat. Nos.2,563,757; 2,719,014; and 3,138,353 are illustrative of prior artutilizing tiltable stabilizer surfaces for the double purposes to moreefficiently provide downward thrust at the rear of the fuselage and tosimultaneously prevent the occurrence of deep stall in an aircraftflying at a low speeds.

Further, the desirability to use small or undeveloped landing zones hasled to the development of aircraft capable of vertical takeoff andlanding (VTOL). To achieve the VTOL feature and still retain relativelyhigh performance in level flight, aircraft have been developed whereonthe wing, or major portions thereof; engines; and, in some designs, theentire stabilizer surface are tiltable in the direction of flight. Theseelements are vertical for landing and takeoff and are horizontal inlevel flight as illustrated, for example, in U.S. Pat. No. 2,621,001.However, the stabilizer surface tilts with trailing edge down unlike theabove-described fixed wing airplanes.

The VTOL airplane with tilting wing and engines suffers from thecomplexities of a dual purpose design and performance compromises whichinevitably occur in attempting to satisfy two such diverse requirementsas vertical and level flight. On the other hand, the more conventionalfixed engine-fixed wing airplane with tiltable stabilizer surfacepreserves level flight performance and provides efficient aircraftcontrol during low speed landings over shortened distances but does fallfar short of effecting a vertical landing or nearly so. What is neededis a substantially conventional airplane having essentially normal levelflight performance and combined with a capability of controlled landingswhich are vertical or substantially vertical.

SUMMARY OF INVENTION

The instant invention comprises the method of flying a generallyconventional airplane having a fixed wing and a fully tiltablestabilizer so that completely controlled and substantially verticaldescent is accomplished while the fuselage of the airplane is maintainedin an orientation similar to level flight. The objective of control isaccomplished by fully stalling the main lift surface, i.e., the wing, ofthe airplane and compensating for the lost wing-lift by a programmedvariation of stabilizer lift and engine thrust.

In another application, the method of this invention controls agenerally conventional airplane having tiltable engine nacelles mountedon a fixed wing so that completely controlled and substantially verticaldescent is accomplished while the wing is in deep stall and the fuselageof the airplane is maintained in an orientation similar to level flight.

OBJECTS OF THE INVENTION

Therefore, it is an object of this invention to provide a method ofcontrolling a generally conventional aircraft having a tiltablestabilizer in a substantially vertical descent while maintaining thefuselage at a desirable attitude to the horizontal.

Another object of this invention is to provide a method of controlling agenerally conventional aircraft having a tiltable stabilizer when theprimary lifting surface is in a continuous deep stall condition.

A further object of this invention is to provide a method of controllinga generally conventional aircraft having tiltable engine nacelles in asubstantially vertical descent while maintaining the fuselage at adesirable attitude to the horizontal.

Still another object of this invention is to provide a method ofcontrolling a generally conventional aircraft having tiltable enginenacelles when the primary lifting surface is in a continuous deep stallcondition.

Other objects, advantages, and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawingin which:

FIG. 1 is a somewhat schematic side elevation view of an airplane havinga tiltable stabilizer.

FIG. 2a and b are simplified vector diagrams of forces acting on theairplane of FIG. 1 during conventional landing.

FIGS. 3a and b are simplified vector diagrams of forces acting on theairplane of FIG. 1 during an uncontrolled deep stall descent.

FIGS. 4a and b are simplified vector diagrams of forces acting on theairplane of FIG. 1 during a controlled deep stall descent.

FIG. 5 is a somewhat schematic side elevation of an airplane havingtiltable engine nacelles attached to the wing tips.

FIG. 6 is a top view of the airplane of FIG. 5.

DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, an airplane 10 suitable for control by themethod of this invention is comprised of a fuselage 12; a fixed wing 14,which is the craft's primary lifting surface; an engine 16 providingforward thrust; and a vertical rudder surface 18. The stabilizer 20 ishorizontal airfoil surface attached pivotally on a horizontal axis 22 atthe rear of the fuselage 12 so that the stabilizer 20 can be orientedwith its chord line above or below the longitudinal axis 24 of thefuselage 12 as required for normal pitch control during flight.Additionally, the stabilizer 20 is capable of extreme tilting, with theleading edge of the stabilizer 20 downward, to angles at least as greatas 65° away from alignment with the fuselage axis 24. Means to tilt thestabilizer 20 are well known, form no part of the present invention, andtherefore are not described herein.

The airplane 10 of FIG. 1, with the stabilizer generally parallel to thefuselage axis 24, is capable of conventional landings, takeoff andflying maneuvers. However, by utilization of the method of thisinvention as described hereinafter, the airplane 10 is capable of acontrolled low velocity descent at angles approaching the vertical,while simultaneously maintaining, the fuselage 12 in a desirable flightattitude, i.e., slightly nose-up from the horizontal 26 (FIG. 1).

To initiate the desired steep descent, the flying speed of the aircraft10 is first reduced by decreasing engine thrust to an air speed justabove stall conditions. Then by lowering the leading edge of thestabilizer 20, the nose of the aircraft 10 is slowly pulled up above thehorizontal 26 until the wing 14 of the aircraft 10 begins to stall.Next, the horizontal stabilizer 20 is tilted at an angle 28 to thefuselage axis 24 between approximately 65° and 80°, with the leadingedge 30 of the stabilizer 20' down relative to the trailing edge. (Theprime marking (') denotes the stabilizer in its extremely tiltedorientation.) Tilt actuation of the stabilizer 20' causes sufficientadditional upward pitch to the wing 14 to bring about complete, deepstall of the wing surface. But as the airplane 10 descends in thedirection of the arrow 34, the stabilizer surface 20' at the extremeangle 28 is unstalled in the relative air stream and provides stablecontrol for the aircraft 10 in all axes. As the aircraft 10 starts itsnearly vertical descent, engine power is increased to compensate forlift which is lost when the wing 14 is stalled and to hold the nose ofthe airplane 10 above the horizontal attitude 26. Thus, a slow, stable,and nearly vertical descent path 34 is maintained. The thrust of theengine 16 may be varied to control the actual rate of descent and theairplane 10 can be steered in the usual manner by operation of therudder 18.

Although it is not known by exactly what mechanism all-axis stability ismaintained during descent of the craft 10 with its main lifting surface14 in deep stall, several theories offer logical bases upon which suchperformance may be anticipated. FIGS. 2-4 present a simplified vectorialanalysis of forces acting upon the aircraft 10 of FIG. 1 in severalstages of flight. Corresponding parts of the airplane bear similarnumbers in all FIGS. 1-4.

FIG. 2a illustrates the airplane 10 in generally horizontal flight atlow speed prior to a conventional landing maneuver. The stabilizersurface 20 is tilted slightly below its normal angle relative to theaxis 24 of the fuselage 12 producing diminished lift 36 at the tail.This pivots the fuselage 12 about its center of gravity 38 to a somewhatnose up attitude. The net force 36 on the stabilizer 20 is upward as aresult of the high velocity air flow from the engine and the forwardmotion of the airplane. The wing 14, not yet stalled, produces lift 40upward and generally perpendicular to the wing chord; and the thrust 42of the engine 16 is forward and parallels the axis 24 of the fuselage12. The net drag of the airplane is shown as a force 44 acting throughthe center of gravity 38 in a direction opposite to the path of travelof the aircraft which path is generally horizontal as indicated by thearrow 46. The weight 48 of the aircraft is shown vectorially actingvertically downward through the center of gravity 38.

A summation of these vectorial forces, FIG. 2b, can produce a closedpolygon 50 indicative of horizontally and vertically stable flight.Also, the counterbalancing moments of wing lift 40 and stabilizer lift36 acting on opposite sides of the center of gravity 38 can providepitch stability. Thus, by proper balancing of these forces, stableflight can be maintained at low speed with the primary lifting surface14 near stall.

When the wing 14 is pivoted to a greater angle of attack, deep stall ofthe wing surface 14 occurs. Wing lift vanishes (FIG. 3a) and thevertical forces are unbalanced (FIG. 3b) leaving a resultant force 51which causes the plane 10 to lose altitude as indicated by the arrow 52.Stabilizer lift force 36 persists and perhaps increases due to a morepositive angle of incidence relative to the air stream as the plane 10descends. Thus, the tail rises unopposed by a counterbalancing wing liftforce and, pivoting about the center of gravity 38, the nose of theairplane 10 falls. This unstable pattern of stall followed by a dippingof the nose is well known in conventional aircraft.

FIG. 4a illustrates the effect of extreme rotation of the stabilizersurface 20' leading edge downward, as prescribed in the method of thisinvention. This operation of the stabilizer 20' is completed to converta near-stall condition (as in FIG. 2) into deep stall of the wing 14 andthereby eliminates the lift force normally produced by the wing 14.Please see FIG. 4a. The plane 10 commences its descent in the directionas indicated by the arrow 54. As before, the weight force 48 actsvertically downward through the center of gravity 38; and the net dragforce 44 acts upward and rearward opposite to the direction 54 of travelof the aircraft.

However, the force 56 produced by the extremely tilted stabilizer 20' isacting downwardly and rearwardly; first, because the air stream from theengine 16 impinges on the upper stabilizer surface 20' rather thanpassing over it; and secondly, because the stabilizer surface 20' nowhas a negative angle of attack relative to the direction 54 of theaircraft's travel. Either force alone might be sufficient to produce thedownward force 56.

It is shown in FIG. 4b that these forces, when combined vectorially, canproduce a closed polygon 58 indicating vertical and horizontal stabilityof the craft 10.

The angle 28 of the stabilizer surface 20' and thus the magnitude anddirection of force it produces are controllable. Also engine thrust 42'is controllable and is increased to overcome the increased tail drag. Soby manipulation of these forces, the operator can in effect close theforce polygon 58 and provide vertical and horizontal stability as theplane moves along its downward path 54. All forces act substantiallythrough the center of gravity 38 of the aircraft 10 thus providing pitchstability as well.

Another theory of operation holds that the aircraft in deep stall istrapped in its own vortex and held in the normal upright altitude. Therate of descent is low due to the resulting high drag of the craft andthe lift created over the surfaces by the vortex flow.

FIGS. 5 and 6 illustrate another type of aircraft 60 suitable forcontrol by the method of this invention. The aircraft 60 is comprised ofa fuselage 62, a fixed wing 64 which is the primary lift surface, avertical rudder surface 66 and a stabilizer surface 68. A pair of enginenacelles 70 are pivotably mounted at opposite wing tips to rotate in avertical plane about a substantially horizontal axis 72. In conventionalflight, the engines 70 are aligned to provide forward thrustsubstantially parallel to the axis 74 of the fuselage 62; and whenpivoted at an extreme angle 76 to the fuselage axis 74, the pivotedengines, shown with broken lines in FIG. 5 and identified by the numeral70', provide an upward thrust. Means to tilt engines are well known,form no part of the present invention, and therefore are not describedherein.

The airplane of FIGS. 5 and 6 with engine thrust generally parallel tothe fuselage axis 74 is capable of conventional takeoff, landing andflying maneuvers. However, by utilization of the method of thisinvention and described hereinafter, the airplane 60 is capable of acontrolled low velocity descent at angles approaching the vertical whilesimultaneously maintaining the fuselage 62 in a desirable, slightlynose-up attitude from the horizontal 26 (FIG 5).

In initiating the desired steep descent, the flying speed of theaircraft is reduced by decreasing engine thrust to an airspeed justabove stall conditions. Then the aircraft nose is pulled up in theconventional manner to initiate stall of the wing 64. Spoilers 78 on thestabilizer 68 are actuated to cause the stabilizer to stall. Loss oflift from the stabilizer 68 causes the tail to drop, further raising theairplane's nose. This induces deep stall of the wing 64; and lackinglift from both wing and stabilizer, the airplane 60 translates downwardthrough a near vertical trajectory. At this time, the engine nacelles 70are pivoted to the upward pointing position, approximately 80° off thenormal line of thrust; and engine thrust is increased or decreased tocontrol the rate of descent. A near-hover condition with attendant softtouchdown is feasible; and conventional steering capability using therudder 66 continues during descent.

Operation of the system can be manual, remote or programmed to seteither type aircraft into the required pre-stall attitude and to actuatethe stabilizer or engine nacelle pivoting mechanism as the aircraftrequires in order to initiate deep stall and then to maintain control inall axes.

It should be understood that the aircraft in FIGS. 1-6 are by way ofillustration and example and are not to be taken as a limitation to thespirit and applications of this invention. For example, the aircraftengines may be of any type, e.g., jet, propeller or reaction motors.Also, the high velocity flow of air from the engine may be released aftof the stabilizer surface rather than forward as illustrated in FIGS.1-6. Additionally, the airplane may be of either high wing or low wingdesign and conventional elevator flaps may be used with the stabilizer20 of FIG. 1 for conventional flight maneuvers.

What is claimed is:
 1. A method for controlling the stalled descent of agenerally conventional airplane having a fuselage with nose and tail, afixed wing as the primary lifting surface, a rudder at the tail for yawcontrol, a power system for generating forward thrust substantiallyparallel to the longitudinal axis of said fuselage, and a generallyplanar fully tiltable stabilizer surface at said tail for pitch control,comprising the steps of:reducing the airspeed of said airplane insubstantially level flight and tilting said stabilizer surface, leadingedge downward, to produce near stall conditions on said primary liftingsurface; further tilting said stabilizer surface with leading edgedownward to induce stalling of said primary lifting surface; furthertilting said stabilizer with leading edge downward at an extreme angleof approximately 65° to 80° with respect to said longitudinal axis ofsaid fuselage to produce deep stall of said primary lifting surface;increasing said thrust of said power system to orient said axis of saidfuselage above the horizontal with said nose higher than said tail; andvarying said engine thrust to control the rate of descent, said rate ofdescent being arrested when said thrust is increased and said rate ofdescent being accelerated when said thrust is decreased.
 2. A method forcontrolling the stalled descent of a generally conventional airplanehaving a fuselage with nose and tail, a fixed wing as the primarylifting surface, a rudder at the tail for yaw control, a stabilizersurface with spoilers at said tail for pitch control, and a tiltablepower system producing forward thrust substantially parallel to thelongitudinal axis of said fuselage during conventional flight andproducing upward thrust at an extreme angle to said axis of saidfuselage during stalled descent, comprising the steps of:reducing theairspeed of said airplane in substantially level flight and operatingsaid stabilizer surface to produce near stall conditions on said primarylifting surface; further operating said stabilizer surface to inducestalling of said primary lifting surface; actuating said spoilers onsaid stabilizer surface to produce deep stall of said stabilizer surfaceand said primary lifting surface whereby descent of said airplaneinitiates; tilting said power system at said extreme angle to said axisof said fuselage to generate upward thrust; varying said upward thrustof said power system to orient said axis of said fuselage above thehorizontal with said nose higher than said tail; and varying said upwardthrust to control the rate of descent, said rate of descent beingarrested when said thrust is increased and said rate of descent beingaccelerated when said thrust is decreased.
 3. The method of claim 2wherein said extreme angle of said power system is approximately 80°. 4.The method of claim 2 wherein said power system comprises a pair ofengines mounted at opposite wing tips of said fixed wing.
 5. A methodfor controlling the flight of a generally conventional airplane having afuselage with nose and tail, a fixed wing as the primary liftingsurface, a rudder at the tail for yaw control, a power system forgenerating forward thrust substantially parallel to the longitudinalaxis of said fuselage, and a generally planar fully tiltable, byrotation about a transverse horizontal axis, stabilizer surface at saidtail for pitch control, comprising the steps of:tilting said stabilizersurface, leading edge downward relative to said longitudinal axis ofsaid fuselage, until the angle of said primary lifting surface relativeto the oncoming air flow is positively increased to approach stalling;further tilting said stabilizer surface with leading edge downwardrelative to said longitudinal axis of said fuselage, until the angle ofsaid primary lifting surface relative to said oncoming air flow isfurther positively increased and stall of said primary lifting surfaceis induced; further tilting said stabilizer with leading edge downwardat an extreme angle of approximately 65° to 80° relative to saidlongitudinal axis of said fuselage, until the angle of said primarylifting surface relative to said oncoming air flow is positivelyincreased such that substantial flow line separation occurs along saidprimary lifting surface causing deep stall of said primary liftingsurface; varying said engine thrust whereby the attitude and flight pathof said airplane is controlled during deep stall of said primary liftingsurface.